It happened in the US long ago for the most part. I often use a tower that is run by someone other than my carrier, but everything just works. This has been the case so long that I can't recall when the last time anyone I know had to think about roaming.

More recently (since 2015) I get free text and data (but not voice) in more than 100 countries around the world. Some countries include voice as well. In short it is coming, if you don't have it ask your carrier why. (odds are it is regulations so get after your representatives and tell them to stop it or you will vote for someone who will)